3 Answers
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Longer answer: Just because something is released as open-source/under the GPL doesn't mean you can't put it behind a paywall. All WP themes have to be licensed GPL if you distribute them, because they are derived from the WordPress code, which is itself GPL, but you can still charge for it.

Your css and images, however, do not have to be released under the GPL - you could do a split license.

If you don't want to distribute your CSS and Images under GPL, you need to distribute them separately from your PHP code (not in one package). That's something Mark Jaquith's article does not cover at all. Rarst wrote that in short: Yes, while you are in line with Twenty Ten license (GPL). So first take a look into the license. And if you're in doubt, you can always ask the original copyright owner(s) who can give you all rights you need anyway.
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hakreFeb 21 '11 at 10:18

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Well, the article does cover that in the FAQ, under What about a developer license? :)
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AmandaFeb 21 '11 at 10:25

well, that FAQ entry does not cover the form of packaging and distribution, it only talks about the license which per-se is not wrong but unless you don't care about the form of distribution you might be dead-wrong to follow that suggestion. Always ask your legal staff for concrete questions if you are uncertain and really want to know because you want to run a business on top of it.
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hakreFeb 21 '11 at 10:31

@hakre Why would one have to distribute CSS and Images (and Javascript too I suppose?) under GPL, if it's included in the same package as the PHP code? Do you have a reference to somewhere where I can read more about that?
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KajMagnusJun 3 '12 at 6:49

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@KajMagnus: That is by the GPL license itself, for example in the license text of GPL-2.0 that ships with Wordpress this in Section 2. That's a common thing with the GPL, and it ensures that re-distribution of GPL'ed software can not be easily prevented, so it protects the software freedom.
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hakreJun 3 '12 at 7:08

You can even sell wordpress and twenty ten unchanged while you got it for free. There is no need for you to create a theme on your own to sell it.

But bottom line is: It depends on many things, so no clear answer can be given to your question.

As far as you are concerned about a legal hint on this, you should ask that question your lawyer with your concrete theme at hand and a concept how you plan to sell and distribute your theme. Next to that you should carry a copy of wordpress and it's license term(s) with you so that it can be checked against. Normally your lawyer can aquire this from the internet directly, but in case not, just take it with you.

In general, the GPL (that's the license WordPress and it's derivates like add-ons often called themes or plugins are distributed under) allows commercial distribution and modification but there are also some terms you need to take care of in respect to the original authors. That's to protect the freedom of the software. It's not only you who is free to sell worpdress, it's everybody and the license has been written to protect this freedom ongoing. That means for the users you distribute under the GPL to, as well.